In manufacturing of a seamless steel pipe by the Mannesmann-mandrel mill process, a round billet heated in a rotary hearth furnace is pierced by a piercer to form a hollow shell, that is, a stock pipe. A mandrel bar with a lubricant applied to the surface is inserted into the shell in a skewering manner, and the shell is rolled into a predetermine dimension by a mandrel mill consisting of 5 to 9 stands at one pass. This is called elongation rolling.
After elongation rolling, the pipe is drawn out with the mandrel bar, cut off a portion with a poor pipe end shape with a hot saw, and reheated in a reheating furnace, and then the outer surface thereof is descaled with high-pressure water. The resulting pipe's outer diameter is successively reduced and the wall thickness is slightly reduced by a stretch reducer, and made into a predetermined product dimension. Thereafter, the pipe is cooled in a cooling bed, cut to a required length by a cold saw, and sent to a shaping line.
During the elongation rolling of the hollow stock pipe by the mandrel mill of the above-mentioned steps, a lubricant is generally applied to the surface of the mandrel mill. The reason for this is that relative slippage is caused between the inner surface of the hollow stock pipe and the surface of the mandrel bar during the elongation rolling, and the hollow stock pipe may stick to the mandrel bar if the interface between both is not sufficiently lubricated, and this may lead to the loss of a product with good inner surface quality. Therefore, the application of the lubricant to the surface of the mandrel bar is performed for preventing the sticking of the hollow stock pipe to the mandrel bar and also for ensuring a stable low friction coefficient.
A lubricant composed mainly of graphite is disclosed in Patent Literature 1, and a lubricant composed mainly of mica is disclosed in Patent Literature 2, these of which have been used.
Further, in recent years, for the purpose of improving the pipes inner surface quality, it has been proposed to apply a lubricant composed mainly of borax to the inner surface of the hollow stock pipe, thereby melting the scale on the inner surface is used, in order to improve the inner surface equality.
Patent Literature 1: Japan Patent Unexamined Publication No. S 50-144868
Patent Literature 2: Japan Patent Unexamined Publication No. S 64-16894
Patent Literature 3: Japan Patent Examined Publication No. H 7-84667
However, even if the lubricant composed mainly of borax is applied to the inner surface of the hollow stock pipe in mandrel mill rolling, the effect cannot be sufficiently exhibited in some cases. Further, the friction coefficient in rolling can be reversely increased, causing an undesirable phenomenon such as deterioration of inner surface quality.